When utilizing photoelectrophoretic imaging technique, colored photosensitive particles are generally suspended in an insulating carrier liquid which is placed between a pair of electrodes are subjected to a potential difference while image-wise exposed. Ordinarily, in carrying out such a process, the imaging suspension is placed on a transparent electrically-conductive plate or electrode in the form of a thin film and light exposure is made through one transparent plate while a second electrode is placed or rolled across the top of the suspension. The ink particles normally bear an initial charge when suspended in the liquid carrier and are therefore first attracted to the transparent plate or base electrode. Upon exposure to a complementary color, however, the particles change polarities by exchanging charge (i.e. electrons) with the base electrode and resulting exposed charged particles then migrate away from the base electrode toward the second electrode, thereby forming complementary images on both electrodes by particle subtraction. For example, yellow pigments selectively absorb blue light, magenta pigments absorb green light, and cyan pigments substantially selectively absorb red light. When a mixture comprising cyan, magenta and yellow particles is image-wise exposed to yellow light, therefore, the cyan and magenta particles become charged and migrate, leaving behind a positive image consisting essentially of yellow particles. Similarly, when a polychromic ink is exposed to a multicolored image, different colored particles absorb light of their complementary color and migrate, leaving behind a full colored positive image on an electrode corresponding to the original multicolored image. An extensive and detailed description of photoelectrophoretic imaging techniques and principles can be found, for instance, in U.S. Pat. Nos. 3,383,993, 3,384,488, 3,384,565 and 3,384,566, and in "Principles of Color Photography" by Hanson et al on page 443.
As noted in or ascertained from the art, the use of mixtures of migrating color particles for photoelectrophoretic imaging purposes is a logical and very practical way of avoiding the expensive and time-consuming conventional processes.
Despite this fact, however, there remains room for improvement both with respect to color balance, stability and tonal response of multi-colored photoelectrophoretic records. In fact, some desirable characteristics appear to be antagonistic, such that a trade off of desirable and undesirable characteristics sometimes becomes necessary in formulating photoelectrophoretic links. For example, exposed magenta ink particles sometimes "charge exchange" or transfer charges to unexposed yellow, cyan, or other magenta particles causing serious errors in electrophoretic response. In addition, there is a tendency for some magenta pigments to absorb both blue and green light. Similar problems can also arise with respect to cyan- and yellow-pigmented ink particles. In addition, the exposure latitude or dynamic range of the colors may be mismatched or insufficient for good color performance. In combination such problems often force the use of less favored ink dyes or pigments to the detriment of overall tonal response.
Clearly, without good control over the "charge exchange" and other problems of the above type, it is very difficult to obtain and preserve high quality color records by using a photoelectrophoretic system.
It is an object of the present invention to improve the flexibility, quality and stability of polychromic photoelectrophoretic images.
It is a further object of the present invention to increase flexibility by broadening the spectrum of color particles, particularly the choice of dyes and pigments which can be utilized for photoelectrophoretic purposes.
It is a still further object of the present invention to improve color record stability and to minimize the interaction (chemical, electrical, and optical) between the classes of color particles utilized in photoelectrophoretic inks.